This invention concerns a screw for apparatuses used for water purification installations, such as aerators or mixers, and more particularly for apparatuses of the type in which a source of power, which may or may not be mounted on a float, drives a shaft on which a screw (impeller device) is fitted. In aerators the water is sucked up, deflected and spread more or less horizontally over the surface of the water. In mixers the water is sucked in and impelled down under the surface of the liquid.
The state of the art impeller device (Belgian patent 884.216) consists of two parts, namely a lower part formed by a screw and an upper part formed by a paddle wheel.
This embodiment was improved by the Belgian patent 893.687 by converting the screw and the paddle wheel to a single body in which the screw is gradually curved in the form of a spiral and merges into the paddles of the paddle wheel. In this way, a screw pump is formed, consisting of one single body in the form of a centrifugal screw propeller.
This screw pump can serve either as an aerator or as a mixer, and can even be combined as a mixer/aerator as described in European patent 366.644.
Such a screw pump can be single-stage (with one helicoidal, spiral-shaped screw blade) but can also be multistage (with several helicoidal, spiral-shaped screw blades). It is clear that a multistage screw involves more frictional losses and causes more turbulence in the flow of water sucked up or sucked in. A multistage pump also demands a greater capacity on the part of the power source. If a single-stage pump is used, the frictional losses and turbulence are reduced, and a power source with lower capacity is sufficient, but the water flow rate is also reduced.